Several routes have already been tested for synthesizing amino acid from unsaturated fatty acids, esters or nitriles involving a metathesis step. However, none of the tested sequences of reactions is truly satisfactory.
According to a first tested route, cross metathesis followed by hydrogenation produces an amino ester containing many impurities. Further, if it comprises more than 9 carbon atoms, the amino ester proves to be very difficult to purify, especially with a distillation yield that is largely insufficient to be able to envisage an industrial application.
According to a second tested route, cross metathesis followed by hydrogenation and then hydrolysis of the amino ester obtained for the purpose of producing an amino acid proves to be unachievable in practice, because it is so difficult to hydrolyze a product that is as reactive as the amino ester, since it has a tendency rather to polymerize under the hydrolysis conditions.
According to yet another tested route, it was realised that cross metathesis involving an acid, such as a fatty acid or acrylic acid, for example, gives such low yields of acid nitrile (<60%) that, even followed by hydrogenation, this type of crossed metathesis does not lead simply to an amino acid.
Moreover, metathesis and/or hydrogenation catalysts have a tendency to entail the spurious formation of secondary amines in parallel with the formation of the primary amine which is the targeted amino acid.
Whereas the amino ester containing 9 carbons in the linear chain is readily purifiable by distillation, for example according to the Pechiney process described in patent FR 1 087 798, this is not the case for heavier amino esters whose main carbon chain comprises more than 9 carbon atoms. The existing processes, especially the abovementioned process, do not make it possible to solve the problem of purification for long-chain amino esters containing more than 9 carbons.
Finally, the presence in an amino acid or amino ester of impurities, even in an apparently small amount of the order of 0.5% to 1%, makes these amino acids or amino esters very difficult or even impossible to use directly as polymerization monomers, especially on account of:                N-alkylation reactions,        low degrees of polymerization (DP) obtained, which do not exceed 30, for example,        
In particular, the presence of impurities, such as secondary amines, induces these DP limitations, but also:                problems of pollution of the polymerized product,        coloration of the polymerized product induced by the impurities (secondary amines, but also ash: nickel, mineral salts, etc.) present in the monomers,        or problems of fouling of the polymerization device.        
The aim of the present invention is thus to synthesize amino acid in high purity, in which the content of impurities, especially of secondary amines, is less than 0.5% by weight and preferably less than 0.2% by weight, by means of a simple process comprising the fewest possible steps, having a high yield of amino acid (>90%) and allowing purification and thus easy polymerization of the amino acid.
The Applicant has now found a process which, in a quite particular sequence of steps, allows this aim to be achieved.